Precision length standard for coherent laser radar

ABSTRACT

This disclosure presents a version of a coherent laser radar system that is immune to environmental effects on Mach-Zehnder-configured interferometers and reference length standards comprising such systems. The present disclosure also describes methods and materials which alleviate interferometer drift due to vapor absorption using a polyimide coated polarization maintaining fiber to reduce the change in the length of a reference arm.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority benefit of U.S. provisional patentapplication Ser. No. 60/662,367 filed Mar. 17, 2005.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention generally relates to Mach-Zehnder-configuredinterferometers and reference length standards in counter chirp FM laserradars. More specifically, methods and materials are described whichalleviate interferometer drift due to vapor absorption observed forconventional fiber jacketing materials

2. Description of Related Art

There are a number of optical systems that can measure a distance to atarget. Such systems typically utilize an open beam propagated throughfree space between the laser source and the target. However, when thetarget location is such that limited free space is available for beampropagation, such systems are of limited use. Thus, well known systemsmay be able to perform distance measurements, but the open beam opticalsensor head prevents application in limited access areas and tightplaces. For example, precision measurement of dimensions inside achassis cannot easily be accomplished with such open beam systems. Whileit is known to transfer light through optical fibers, precision iscompromised due to the environmental effects on the fiber itself. Theseenvironmental effects can change the optical path length and thepolarization of the light in the fiber, adversely affecting measurementprecision.

The existing art in precision FM laser radar incorporates a single chirplaser source and a polarization maintaining fiber optic geometry withseparate local oscillator (LO) and signal paths (see, e.g., U.S. Pat.Nos. 4,824,251 and 4,830,486). Such laser radars most typically use alength standard of some kind as a basis for a high-precision absolutelength measurement.

The existing art in length standards for these and related applicationsfall into two primary categories. One category involves an artifactbased standard such as temperature compensated metal or glass bars/tubeswith targets mounted to each end. For the range needed (2-4 m) these areunwieldy and it is impractical to integrate them into a measurementsystem. The second category utilizes a fiber optic based lengthstandard, which can easily be packaged into a small volume.

In an improvement on the single chirp laser system referred to above,the inventor herein has developed a counter chirp configuration thatprovides a much greater insensitivity to vibration induced range errorsby providing for a more accurate Doppler correction. Moreover, bycombining the LO and signal paths for two lasers into a single fiber,the fiber optic circuit is both less complicated and less expensive dueto fewer components and completely immune to error caused by changes inthe LO and signal path lengths due to environmental factors such astemperature. This configuration is described in U.S. patent applicationSer. No. 11/354,382, filed Feb. 15, 2006, the contents of which areincorporated herein by reference as though set forth in their entirety.That technology utilized reference standards either of the Mach-Zehndertype or of the Michelson type. However, that application did notdisclose methods of preventing non-temperature-related drift.

Ahmadvand et al. (U.S. Pat. No. 6,778,278) disclose a method tocompensate for temperature drift in a Mach-Zehnder interferometer byadding jacketing material of a specific thickness and length to one orboth arms of the interferometer as a means of eliminating the need foractive temperature control of the device. The result is that the twoarms change the same amount with temperature changes despite the factthat they are different lengths. However, for higher channel densityapplications differences in path length will increase (as would the needfor the temperature compensation). Moreover, they do not address otherenvironmental causes of interferometer drift.

Bauer et al. (U.S. Pat. No. 6,757,469) disclose fabrication methods ofwaveguide devices that insure temperature insensitivity. These waveguidedevices are not optical fibers. Again, the disclosure is limited totemperature effects not alleviation of vapor absorption effects.

Typically, the Mach-Zehnder interferometer formed by two couplers andthe fiber between them is kept in a temperature-controlled container toprevent the fiber lengths from changing. If the difference in fiberlengths is calibrated, the reference interferometer can serve as anabsolute length standard for the laser radar system as well as provide asignal useful in the linearization of the laser waveform.

FM lasers are largely immune to ambient light conditions and changes insurface reflectivity because FM laser radars rely only on beatfrequency, which is not dependent upon signal amplitude, to calculaterange. This enables the FM Coherent system to make reliable measurementswith as little as one picowatt of returned laser energy. Thiscorresponds to a nine order-of-magnitude dynamic range of sensitivity.However, these instruments are not immune to drift caused by otherambient conditions.

Ambient conditions other than temperature can affect effective opticalpath lengths in several ways. In particular, the presence of water vaporin the surrounding air affects the optics. In addition to water vapor,volatile components of the polymers (especially acrylate) can outgas,thereby causing a dimensional change in the jacket and thus a change inthe fiber as described below.

An effective optical path length in an optical fiber can change eitherbecause of a change in the refractive index of the core of the fiber,potentially caused by absorption of impurities, or because of a physicalchange in diameter or length of the fiber brought about by ambientconditions. If a fiber's jacket absorbs, for example, water vapor orother impurities, the jacket's dimensions change, thus exertingcompressive force on the underlying glass fiber. Conversely, outgassingof water vapor or other impurities diminishes compressive force on theglass fiber. The change in force causes both the diameter and length ofthe fiber to change by small amounts and possibly the index ofrefraction to change, causing a change in effective optical path length.Since the precision being sought in the current invention is on theorder of one part per million, it can be seen that tiny changes indimension or optical path length can affect the precision of theinstrument. The change in force can also degrade the polarizationmaintainability of the fiber due to stress birefringence.

What is needed is a jacketing/coating material that does not tend toabsorb or outgas water vapor and other impurities which can cause aneffective change in the length of the reference arm.

SUMMARY OF THE INVENTION

Acrylate is the principal heritage coating or jacketing material foroptical fibers. It has been designed into flight grade optical fiber foroptical cable since the 1980's. The advantages of the acrylate materialis that it can be selected for strippability (for example, withmethylene chloride) and is fairly soft, which makes the fiber moreflexible. Its disadvantages are that acrylates tend to have lowtemperature ratings (usually around 85° C although some are availablethat are rated as high as 200° C.) and it is a well known outgasser.

During development of the invention disclosed in U.S. patent applicationSer. No. 11/354,382, filed Feb. 15, 2006, the inventors determined thatdrift in the instrument was causing a loss of precision over that deemedto be achievable. Investigation and analysis showed that lengthvariations of the acrylate-coated fiber was at fault, and it wasdetermined that vapor absorption and outgassing of the acrylate were theroot causes of optical path length variations.

Once the cause of the length variation of the acrylate coated fiber wasisolated, several options were proposed. Initial attempts to limit theeffect of acrylate proved ineffective, so it became necessary to testexisting systems with alternate fiber coatings, specifically low meltingpoint metal and polyimide.

The present invention makes use of specially coated polarizationmaintaining (PM) fiber to reduce the change in the length of thereference arm. In the most preferred embodiment, the coating ispolyimide. An example of polyimide coated fiber available commerciallyis to be found at http://fibercore.com/05hb-hbp.php, “HB-P PolarizationMaintaining Fiber.” As can be inferred from the contents of this link,until the present invention, polyimide coating has been used primarilyfor high temperature performance, e.g., to withstand medicalsterilization and high curing temperatures. What is most novel in thecurrent invention is to use polyimide and other acceptable coatings toprovide dimensional, optical path length, and birefringence stability inaddition to high temperature performance.

However, other coatings are acceptable substitutes. Other coatings canperform the same function. Fibers are available and have been jacketedwith many polymer coatings including Ultraviolet Cured (UV-cured)Acrylate, Polyimide, Teflon, DuPont Tefzel, Hytrel and Nylon. Afteracrylate, polyimide is probably the most readily available off-the-shelfespecially in PM fiber and works well for this application. In addition,metal coated fibers can provide dimensional stability but may havehigher temperature sensitivities. Fibers that function in the waydescribed in this disclosure are referred to herein as substantiallyhydrophobic and substantially non-outgassing.

Polyimide has been used successfully for coating space grade opticalfiber and it comes with the advantage of a 125° C. temperature rating.However, it was not an obvious choice for use in a laser metrologydevice because of its known disadvantages. Polyimide has exhibited somedisadvantages that have made it a less than desirable first choice forcoating optical fibers. For example, polyimide tends to make the fiberseem stiffer than an acrylate coated fiber. Second coatings have beenadded, but the known additional coatings come with their own problemsthat have limited the desirability of polyimide for mainstreamapplications. For example, such second coatings have exhibitedoutgassing and contamination problems. Of even more significance are thedifficulties associated with stripping polyimide. Polyimide coating canbe stripped chemically, with hot sulfuric acid, or mechanically, using ahot tweezers. Users find hot sulfuric acid to be dangerous andnon-portable. Mechanical stripping methods are not recommended becausethey have the capacity to introduce surface flaws to the glass fiber,compromising long term reliability. As noted elsewhere, the realizationthat it offered superior vapor absorption and outgassing properties thatprovided improved dimensional stability led to its adoption for thecurrent invention.

The coherent laser radar as it is currently envisaged requires a fiberoptic reference arm interferometer in a Mach-Zehnder configuration asits reference length standard. This internal length standard iscalibrated by using an external interferometer to determine the exactlength differential between the two paths of the referenceinterferometer. In a related aspect, if this length difference varieswith time there will be a corresponding scaling error in rangemeasurements on the measurement path. In one embodiment, the presentinvention diminishes the length variation within the reference arm,allowing for long-term range accuracy without the need for frequentcalibration.

The present invention is applicable to all non-contact precisiondistance measuring applications that incorporate a fiber optic lengthstandard. The insensitivity of the polyimide coating to environmentalconditions allows a system to operate for months in a changingenvironment and remain within specification without the need for a rangecalibration, effectively reducing the necessary system maintenance andextending the time between scheduled certification.

Exemplary methods and systems according to this invention are describedin greater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the reference arm.

FIG. 2 shows a metal coated fiber test setup.

FIG. 3 shows a dual Zerodur test setup.

FIG. 4 shows a prototype polyimide reference arm.

FIG. 5 show a polyimide coated reference arm with dual Zerodur LOs.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is also to be understood that the terminology used herein is for thepurpose of describing particular embodiments only, and is not intendedto limit the scope of the present invention. It should be furtherunderstood that as used herein and in the independent claims, thesingular forms “a,” “an,” and “the” include plural reference unless thecontext clearly dictates otherwise. Thus for example, reference to “anisolator” includes a plurality of such isolators, reference to a“reference arm” is a reference to one or more arms and equivalentsthereof known to those skilled in the art.

As used herein, “coherent radiation,” including grammatical variationsthereof, means radiation in which the phase relationship between any twopoints in the radiation field has a constant difference, or is exactlythe same in either the spatial or the temporal mode throughout theduration of the radiation. In particular, a laser is a device whichartificially generates coherent radiation in the form of light within anarrow range of wavelengths.

In order to understand the current fiber optic based length standard itis necessary to have some grounding in FM laser radar in general. Incoherent, or FM laser radar that uses a diode laser as its source, thefrequency of the laser is modulated directly by modulating the laser'sinjection current. The mode of obtaining a precise relative distancemeasurement in a state of the art system is spelled out in detail U.S.patent application Ser. No. 11/354,382, filed Feb. 15, 2006, thecontents of which have already been incorporated by reference herein.

Advanced techniques employed in the invention set forth in U.S. patentapplication Ser. No. 11/354,382 enable a high degree of linearity in therelative distance measurements. In addition, these techniques can detectand compensate for real time variances from linearity. These featuresenable range measurement with a single digit micron precision. Suchprecision is high enough that preventing drift caused by environmentalfactors becomes extremely important.

For precision measurements with an FM laser chirp system it is necessaryto include a reference standard both for absolute ranging accuracy andto help linearize the laser's chirp waveform. As mentioned above, thisreference standard can take the form of a fiber optic interferometer ina Mach-Zehnder configuration, 10, FIG. 1. For the internal referencestandard, the light from an IR laser, 101, after passing through anisolator (not shown), is split into two fibers by means of a fibercoupler, 102. One path is used to measure range, 103. The other path isdirected to the reference standard that consists of an input fiber opticcoupler, RC1, 104, which splits the light into two fiber paths (105 a,105 b) and an output fiber coupler, RC2, 106, which recombines the lightinto a single fiber. The two paths of fiber between the two couplers aremismatched in length by several meters such that a laser radar signal isdetected on the detector, R1, 107.

Typically, the Mach-Zehnder interferometer formed by the two couples andthe fiber between them is kept in a temperature-controlled container toprevent the fiber lengths from changing. If the difference in fiberlengths is calibrated, the reference interferometer can serve as anabsolute length standard for the laser radar system as well as provide asignal useful in the linearization of the laser waveform. Thisdifference in fiber length is typically set to an equivalent range of 4meters in air and the laser is typically linearized to give a 2.3 MHzbeat frequency over the reference arm path. Knowing that the referencelength is calibrated, it is then easy to calculate the distance from thescanner to any point in space by using a scaling factor calculated by:Scaling (MHz/M)=2.3 MHz/Δd Reference (˜4 m)   (5)

Typically, the single mode, polarization maintaining fiber widelyavailable uses a UV cured acrylate coating that is hygroscopic. Thiscoating tends to absorb or outgas water vapor and other impuritiescausing an effective change in the optical length of the reference arm.In many cases, this drift can be severer and rapid, resulting in achange of several hundred microns (10-100 parts per million (PPM)) in amatter of hours/days. Unlike drift in one of the LO paths which simplytranslates into a range offset, this change in the reference arm lengthhas a direct effect on the range accuracy of the machine, i.e., anincrease in reference arm length of 1 part per million results in anacross the board range error of 1 part per million. This can be easilyseen by comparing a calibrated reference arm path of exactly 4 meters toone that has increased by 40 microns or 10 PPM. These values give ascaling value of 0.575000 and 0.57499425 MHz/m respectively. Thus, foran actual range differential of 10 meters, the drifted value will be 100microns long.

This application discloses use of specially coated PM fiber to reducethe change in the optical path lengths in the reference arm. Whilepolyimides have numerous uses in several fields, as a fiber opticcoating it offers a reduced cross sectional area (low profile) thatwould also be heat resistant for high temperature applications such asmedical sterilization and high curing temperatures of various laminates.It is also used for increased protection in hostile environments. Thecurrent application makes use of its water resistant nature (typicallyused only in textiles; see, e.g., Hope, Diane, “New Technologies forFire Resistant Textiles and Membranes,” Innovation, vol. 8, no. 5,September/October 2000) to eliminate the outgassing common in theacrylate coated fiber.

The invention will now be described in greater detail by reference tothe following non-limiting examples. The following examples are intendedto illustrate but not limit the invention.

EXAMPLES

During the production process, the 50/50 fiber optic couplers are fittedwith output fibers coated in polyimide. When the reference arm is built,only fiber coated with polyimide is used, resulting in a reference armwhose drift will be minimized due to the absorption on or outgassing ofthe fiber jacket. The thin polyimide coating exhibits a markedimprovement over the acrylate, resulting in an order of magnitudeimprovement over the old technology. The improved performance can beeasily demonstrated by comparison with the calculation above. With theimproved coating, the reference path increases by 4 microns, or 1 PPM.This gives scaling factors of 0.575000 and 0.574999425 MHz/m. At the10-meter range, the drifted value is only 10 microns from the assumedvalue in comparison to 100 microns before, or an improvement of about afactor of 10 in precision over the conventionally coated fiber.

In order to test the relative range drift of an MV200™ (i.e., portablecoherent laser radar system sold under the Trademark MV200™) referencearm built with different fiber optic coatings, a test setup was built ina temperature-controlled environment, 12, as shown in FIG. 2. Initially,the setup consisted of a current MV200™ oven, 121, used as a lasersource feeding into a test oven that contained a standard reference arm.The fiber within the oven was coated as thoroughly as possible with lowmelting point metal, 123. The signal generated by the test oven wasmeasured by the detector.

To monitor the range drift, light was split from the laser source usinga 90/10 coupler, 122. This light was sent, via a 70/30 coupler, 124, toan open beam Michelson interferometer consisting of a fiber pigtailattached to a fiber mount and a lens that would send a collimated beamto a mirror and then a retroreflector, which would then return the beamback to the pigtail. Reflected light from the pigtail acted as theinternal LO and mixed with the reflected light from the retroreflectorto generate the radar signal which was measured by the detector, 125,via the coupler, 124.

The components were mounted on a length of ZERODUR® glass, a substancethat exhibits minimal thermal expansion (±0.15×10⁻⁶/degree C.). Manyoptical applications require a substrate material with a near-zerocoefficient of thermal expansion and/or excellent thermal shockresistance. ZERODUR®, with its very small coefficient of thermalexpansion at room temperature, is such a material. ZERODUR, whichbelongs to the glass-ceramic composite class of materials, has both anamorphous (vitreous) component and a crystalline component. This Schottglass is subjected to special thermal cycling during manufacture so thatapproximately 75% of the vitreous material is converted to thecrystalline quartz form. The crystals are typically only 50 nm indiameter, and ZERODUR appears reasonably transparent to the eye becausethe refractive indices of the two phases are almost identical. However,scattering at the grain boundaries precludes the use of ZERODUR fortransmissive optics.

This formed an open-air path along the zerodur length that allows for aconstant, unchanging range measurement. FIG. 2 shows the complete layout(12). Thus, the drift in the test oven was measured by comparing therange measurement at the test oven detector, 127, to the range measuredat the monitor detector, 125.

This setup was attached to standard MV200™ M signal processingelectronics with scanner control flags disabled. The setup was poweredon, linearized, and allowed to settle for about an hour. Data wascollected over the course of a week by using the stare measurementdialog, and all data was corrected for range deviation due toenvironmental temperature and pressure changes.

In order to test the stability of the zerodur path interferometer, thesetup from FIG. 2 was altered such that the test oven was removed and asecond zerodur path interferometer installed, 13, as shown in FIG. 3.One of these paths was used as a reference arm and the other was used aslocal oscillator one (LO1) and data was recorded over the course of aweek. The light from the MV200™ oven, 131, passes through a 50/50 opticcoupler, 133, thereby splitting the beam into two fibers. The splitbeams are sent via the 70/30 couplers 135 and 136, to the open air pathMichelson interferometers, 137, as described above. The output of eachinterferometer is measured by the detectors, 132 and 134. By monitoringthe output of the detectors, the intrinsic drift of the zerodur pathscan be measured.

Another test oven was then built with polyimide-coated fiber replacingthe metal/acrylate coating, 14, FIG. 4. This was accomplished by cuttingthe internal leads of the 50/50 couplers, 141 and 142, as short aspossible and replacing the bulk of the fiber with polyimide coated fiber(143 b). Instead of exactly repeating the test done with themetal-coated fiber, both zerodur arms were used as LO1 and LO2 while thepolyimide oven was used as the reference arm as shown in FIG. 5.

The light from the MV200™ oven, 151, passes through a 90/10 opticcoupler, 152, splitting the beam into two fibers. A first fiber beam ispassed through a 50/50 optic coupler, 153, thereby splitting the firstbeam into two separate fibers which are sent, via the 70/30 couplers,154 and 155, to the open air path Michelson interferometers, 158, asdescribed above. The output of each interferometer is measured by thedetectors, 156 and 157.

The second beam is sent through a polyimide coated reference arm 160,which is mounted on a thermal electric cooler (TEC), 161, that iscontrolled by a TEC controller, 162. The beam is then sent to adetector, 159. Data was taken as before.

For the final test, the same set-up was used, but the standard MV200™Oven number 6 was used as LO1 and one of the zerodur paths was used asLO2 (FIG. 5). This data was taken for three days.

The data was tabulated and the range was corrected for changes in theenvironment. The range spread was then calculated, as was the spread inPPM. These results are summarized in Table 1: TABLE 1 Test results frominitial coatings testing (Set 3) (Set 3) (Set 4) (Set 4) Metal CoatedZerodur Polyimide Polyimide Polyimide Polyimide fiber vs. reference vs.fiber vs. fiber vs. fiber vs. Oven fiber. vs. Zerodur Zerodur ZerodurLO1 Zerodur LO2 6 reference Zerodur Range Spread 52.9 11.7 15.2 18.669.5 18 in Microns Range Spread 27.3 5.0 7.1 8.7 19 7.9 in PPM

It is clear that the polyimide coated fiber performed substantiallybetter than that coated with acrylate. In fact, the polyimide fiberperformed almost as well as zerodur compared to zerodur, suggesting thatthe drift within the polyimide reference could possibly be attributed todrift within the relatively short acrylate leads or to inherentinstability of the open air setup.

At this point, 50/50 couplers were produced with polyimide leads toensure that the critical lengths (i.e., where the reference path wassplit) would exhibit maximum stability. These couplers were used tofabricate a reference arm in a standard MV200™ oven and mounted in asystem to be tested as a complete unit. This testing consisted of astandard Range Calibration and Verification followed by scannerperformance testing under a variety of environmental conditions. Thescanner was then subjected to another Range Verification. A second RangeCalibration was performed if necessary.

Sine the first polyimide reference arm performed well, several more wereintegrated into subsequent ovens. Table 2 summarizes the range driftperformance of these ovens as compared with those of acrylate leads.TABLE 2 Acrylate and Polyimide Reference Arm range drift afterenvironmental testing Acrylate Polyimide PPM change after testing 15.51.08

Thus, what has been described is an improved jacketing/coating materialthat does not tend to adsorb water vapor or outgas impurities which cancause an effective change in the optical path length of elements of areference arm. While the invention has been described in connection withwhat is presently considered to be the most practical and preferredembodiments, it is to be understood that the invention is not to belimited to the disclosed embodiments, but, on the contrary, is intendedto cover various modifications and equivalent arrangements includedwithin the spirit and scope of the appended claims. The scope of theappended claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures.

1. A substantially hydrophobic and substantially non-outgassing coatingfor optical fibers, whereby the optical fiber maintains substantialdimensional and refractive stability.
 2. The coating of claim 1 whereinthe coating is a polymer.
 3. The coating of claim 1 wherein the coatingis selected from the group consisting of: Ultraviolet Cured Acrylate,Teflon, DuPont Tefzel, Hytrel, and Nylon.
 4. The coating of claim 1wherein the coating comprises a metal.
 5. The coating of claim 2 whereinthe polymer is polyimide.
 6. A method of preserving optical path lengthand polarization maintaining properties of optical fibers comprisingcoating an optical fiber with a substantially hydrophobic andsubstantially non-outgassing coating.
 7. The method of claim 6 whereinthe coating is a polymer.
 8. The method of claim 6 wherein the coatingis selected from the group consisting of: Ultraviolet Cured Acrylate,Teflon, DuPont Tefzel, Hytrel, and Nylon.
 9. The method of claim 6wherein the coating comprises a metal.
 10. The method of claim 7 whereinthe polymer is polyimide.
 11. In a fiber optic interferometer, theimprovement comprising at least one optical fiber coated with asubstantially hydrophobic and substantially non-outgassing coating,whereby the optical fiber maintains substantial dimensional andrefractive stability.
 12. The improvement of claim 11 wherein the fiberoptic interferometer is a reference standard in a laser distancemeasuring apparatus.
 13. The coating of claim 11 wherein the coating isa polymer.
 14. The coating of claim 11 wherein the coating is selectedfrom the group consisting of: Ultraviolet Cured Acrylate, Teflon, DuPontTefzel, Hytrel, and Nylon.
 15. The coating of claim 11 wherein thecoating comprises a metal.
 16. The coating of claim 13 wherein thepolymer is polyimide.